PROJECT SUMMARY/ABSTRACT Despite major investments, the promise of immunotherapy for Systemic Inflammatory Response Syndrome (SIRS) remains unfulfilled. A hyperinflammatory SIRS subset suffers much higher mortality, and thus is in greatest need of new therapies to limit immunopathology. The clinical factors used to define hyperinflammatory SIRS were inspired by a life-threatening complication of pediatric rheumatic disease called Macrophage Activation Syndrome (MAS). Recently, MAS was genetically associated with hyperactivity of the NLRC4 inflammasome and excess Interleukin (IL)-18, strongly suggesting that excess IL-18 is a fundamental host susceptibility factor for hyperinflammatory SIRS. The objectives of this proposal are to identify the relevant mechanisms driving IL-18 overproduction and to define the pathways by which IL-18 promotes hyperinflammation. Our central hypothesis is that intestinal IL-18 overproduction promotes both lymphocyte hyperactivation and dysfunction to drive hyperinflammation and the MAS phenotype. Our guiding rationale is that understanding the mechanisms driving hyperinflammatory SIRS will enable rational identification of at-risk patients, identify potential prevention strategies, and facilitate the use of targeted immunotherapies necessary to prevent organ dysfunction and death. To accomplish our objectives, we first aim to identify the causes of elevated IL-18 in MAS. This will involve (i) determining the molecular machinery necessary for inflammasome-driven intestinal overproduction of IL-18; (ii) isolating the specific microbial factors affecting intestinal IL-18 production and testing their effects on experimental MAS; (iii) determining the cellular sources (intestinal vs. myeloid) of inducible IL-18 in models of systemic hyperinflammation; and (iv) correlating systemic IL-18 levels with fecal colonization in children with Systemic Juvenile Idiopathic Arthritis, who are at high risk for MAS. Our second aim is to define the mechanisms by which IL-18 promotes hyperinflammation. This will involve (i) determining the immunologic mechanisms by which IL-18 promotes models of hyperinflammation; (ii) determining how cytotoxic impairment and excess IL-18 synergistically promote immunopathology; and (iii) determining how NLRC4 hyperactivity and excess IL-18 promote susceptibility to infection. These experiments will contribute significantly to defining new genetic and mechanistic drivers of hyperinflammation, and they will guide precision diagnostics, prevention strategies, and targeted treatments to prevent morbidity and mortality in hyperinflammatory SIRS. The proposed research is innovative because it originates with mechanisms derived from monogenic human hyperinflammatory diseases to understand and manipulate SIRS physiology more broadly. Ultimately, we expect completion of the proposed studies to provide specific insights useful for guiding early detection of the hyperinflammatory SIRS phenotype and novel means to disrupt life-threatening immunopathology.